1. Field of the Invention
The invention relates to a lens arrangement comprising a laterally displaceable optical axis for particle beams, in particular for transmitting regions of an object plane into the image plane by means of electrons, comprising a combined lens, which consists of a cylinder lens and a quadrupole lens, which has slit diaphragms which can be acted on by electrical and/or magnetic fields, the optical axis of the quadrupole lens extending in the centre plane of the cylinder lens, and defining the optical axis of the image, the position of the optical axis of the quadrupole lens being variable in the direction of the slit-shaped aperture of the cylinder lens, the focusing of the quadrupole lens taking place in the section in which the cylinder lens is unfocused, and the defocusing of the quadrupole lens taking place in the section in which the cylinder lens is focused.
2. Description of the Prior Art
In the production of microscopically fine structures, such as the production of semiconductor devices or integrated circuits, it is necessary to permanently monitor the production process as regards the occurrence of process defects. The aim of this monitoring is the early recognition of defects in the surface structure of the manufactured products and, if appropriate, to take appropriate countermeasures. For monitoring, according to the prior art, microscopy methods are employed, by means of which the surface structure is imaged and optically monitored. Light-optical and electron-optical methods are customarily used, the latter method providing the advantage of substantially higher resolution by virtue of the substantially smaller wavelength of the electrons. This means that the distance between closely adjacent points of an object can assume smaller values in the electron-optical method before the two points in the image plane of the imaging arrangement can no longer be separated as individual points.
In the monitoring of the surface structure, methods are preferred which allow both detection of defects and, when defects are recognised, detailed investigation. In the first of these cases, attention is particularly directed towards a high throughput matched to the production rate in order to allow analyses during production. The electron-optical system must correspondingly allow imaging of a largest possible object field with moderate resolution capacity. In the second case, on the other hand, an image with high optical resolution and good contrast is in particular required.
According to the prior art, low-voltage electron emission microscopes with rotationally symmetrical immersion objectives are used for detailed investigation of the aforementioned surface structures. Here, the specimen to be investigated is illuminated within an object field with low-energy electrons. The secondary electrons released in this process are filtered from the surface of the object by means of an immersion objective and imaged with comparatively high resolution in the image plane. It is customary to use energy filters here to restrict the imaging to electrons from the low-energy portion of the secondary electron spectrum in the range from 1 to 2 eV. In this range, the electron yield is high and the energy window can thus be chosen so narrow that imaging with high resolution is permitted with an adequate aperture angle.
With known microscopes of the aforementioned kind, however, it is regarded as disadvantageous that the image is restricted to a very small object field of typically about 100 μm diameter in the direct vicinity of the optical axis. This leads to the effect that objects of large extent, such as the wafer regions with a dimension of approx. 25 mm×25 mm used in the production of integrated circuits, must be scanned in two spatial dimensions if their entire surface is to be imaged. Conventional systems are therefore equipped with a slit for receiving the workpiece, which is displaceable perpendicular to the optical axis in two coordinate directions. The two-dimensionally traversable slide with the required high precision, however, is firstly very complicated to construct, and correspondingly expensive to manufacture. Secondly, by virtue of the comparatively low traverse rate, it limits the efficiency of the apparatus.
The prior art also includes electron-optical systems in which elements with non-circular fields are used.
A device of this kind is disclosed in DE 196 34 456.5 (=Spehr/Kerkhoff). It has an electrostatic cylinder lens and a magnetic quadrupole, which, in combination, form images stigmatically like a circular lens. On the quadrupole can be superimposed a magnetic dipole field, which allows a displacement of the optical axis of the image in the direction of longitudinal stretching of the cylinder lens gap.
The second device of the aforementioned type is disclosed in DE 196 44 857.4 (=Rose/Schmidt). It includes an electrostatic cylinder lens comprising at least three electrodes, of which the centre electrode in the longitudinal direction of the slit-shaped electrode aperture consists of segments insulated electrically from one another (comb electrodes). By virtue of this construction, different potentials can be applied to the individual segments of the centre electrode. In this device, the potential distribution is chosen such that an electrostatic quadrupole is generated, whose axis can be displaced in the direction of the gap.
Both of the aforementioned devices are conceived as electron-optical individual lenses and are used as probe-forming arrangements in shaped-beam exposure systems. These systems are less suited for imaging secondary electrons released in an object plane.
Finally, DE 101 36 190.4 discloses a system in which two non-circular systems of the aforementioned type are used. The system permits the error-free projection of, in each case, comparatively large regions of a transmission mask located in the object plane onto a wafer located in the image plane. Here, the optical axis of the quadrupoles, and thus the optical axis of the image is displaced by means of electrical or magnetic fields over the full width of the wafer region, the projection system maintaining its imaging properties. Correspondingly, with this system for describing the entire wafer region only a mechanical displacement of the object in one coordinate direction is required.
The above-described system is only suitable for projection of masks. However, its disadvantage consists in the fact that it is unsuitable for secondary electron imaging and only permits a comparatively low enlargement. Furthermore, for the aforementioned system, it is essential to the invention that two lens systems are used. The entire system is therefore comparatively complicated and correspondingly expensive to manufacture.